Synchronous electric motor system

ABSTRACT

The present invention aims to provide a synchronous motor drive system that is capable of suppressing ripples in current while reducing switching loss. The system includes three-phase inverters  201 - 203,  a control circuit  400  for controlling the operations of the three-phase inverters and a synchronous motor  300  including a plurality of three-phase coils. To control the operations of the three-phase inverters, the control circuit  400  causes the three-phase inverters  201  and  203  and the three-phase inverter  202  to use different carrier frequencies to generate three-phase AC power, and each of the three-phase inverters supplies a different one of the three-phase coils with three-phase AC power.

TECHNICAL FIELD

The present invention relates to synchronous motor drive systems, andparticularly to technology for controlling inverters for supplyingdriving power to synchronous motors.

BACKGROUND ART

In the use of a synchronous motor, it is required to boost the totalefficiency of a synchronous motor drive system for driving thesynchronous motor. To achieve a high efficiency, technology called PulseWidth Modulation control (hereinafter referred to as “the PWM control”)is often used in three-phase inverters for supplying driving power to asynchronous motor, for example.

Meanwhile, due to development of semiconductor technology, synchronousmotors have been commonly used as vehicle motors in recent years, inview of their reliability, controllability, efficiency, etc., eventhough the power source of a vehicle motor is a battery, which is a DCpower source. In such cases, a high torque is required as well as a highefficiency.

Generally, to increase the torque of a synchronous motor, it isnecessary to increase the frequency of the motor current. In the case ofa three-phase inverter performing the PWM control, however, thefrequency of switching operations also increases as the currentfrequency increases. This increases the switching loss, which isproblematic.

To decrease the switching loss, Patent Literature 1 and PatentLiterature 2 disclose technology for driving a three-phase inverter witha low carrier frequency while the rotation speed is low, and driving thethree-phase inverter with a high carrier frequency while the rotationspeed is high, for example. Such technology reduces the switching loss,which is problematic in increasing the frequency of an inverter,according to the drive state of a motor.

CITATION LIST Patent Literature

-   [Patent Literature 1] Japanese Patent Application Publication No.    2002-153096-   [Patent Literature 2] Japanese Patent Application Publication No.    2001-186787

SUMMARY OF INVENTION Technical Problem

With the technology disclosed in Patent Literatures 1 and 2, although itis possible to reduce the switching loss when the rotation speed is low,it is not possible to reduce the switching loss when the rotation speedis high. Furthermore, ripples in the current are large when thethree-phase inverter is operated with a low carrier frequency. Thiscauses a problem that the ripples cause the motor to vibrate and make anoise.

In view of the above problems, the present invention aims to provide asynchronous motor drive system that is capable of suppressing ripples incurrent which cause a motor to vibrate and make a noise, while reducingthe switching loss.

Solution to Problem

To achieve the aim, the present invention provides a synchronous motordrive system comprising: three-phase inverters each configured toconvert DC power to three-phase AC power; a control circuit configuredto control operations of the three-phase inverters; and a synchronousmotor configured to include three-phase coils supplied with three-phaseAC power from the three-phase inverters, wherein the three-phaseinverters include first and second three-phase inverters, and thecontrol circuit controls the operations of the three-phase inverters bycausing the first and the second three-phase inverters to use differentcarrier frequencies from each other to generate three-phase AC power,each of the first and the second three-phase inverters supplies adifferent one of the three-phase coils with three-phase AC power, thesynchronous motor has a stator in which stator coils are arranged alonga rotation direction of the synchronous motor, each of the stator coilsconstitutes one phase of one of the three-phase coils, and the statorcoils include a first stator coil and a second stator coil arrangedadjacent to each other, the first stator coil constituting one phase ofone of the three-phase stator coils that is supplied with three-phase ACpower from the first three-phase inverter, the second stator coilconstituting said one phase of another one of the three-phase statorcoils that is supplied with three-phase AC power from the secondthree-phase inverter.

Advantageous Effects of Invention

With the structure described in Solution to Problem above, thesynchronous motor drive system pertaining to the present invention iscapable of determining a different carrier frequency for each of thethree-phase inverters in performing PWM control on a single motor withthe plurality of three-phase inverters. The synchronous motor drivesystem determines the carrier frequency of at least one of thethree-phase inverters to be lower than the others to cause the ripplesof the motor currents output from the three-phase inverters to interferewith each other, so that the ripples of the motor current output fromthe three-phase inverter that operates with the lower carrier frequencycan be suppressed. The three-phase inverter that operates with the lowercarrier frequency also contributes to reduce the switching loss.

Thus, the present invention is capable of suppressing motor currentripples that cause the motor to vibrate and make a noise, while reducingthe switching loss which is a problem arising along with the increasedfrequency of the inverters. The present invention provides a synchronousmotor drive system that realizes high efficiency, low noise, low EMC andhigh reliability.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows an overall structure of a synchronous motor drive systempertaining to Embodiment 1.

FIG. 2 is a plan view of a synchronous motor 300.

FIG. 3 shows the details of the synchronous motor in FIG. 2.

FIG. 4 shows connections of stator coils in the synchronous motor inFIG. 2.

FIG. 5 shows an internal structure of a PWM control unit 401.

FIG. 6 shows an example pattern of a gate control signal generated by aPWM signal generation circuit 414.

FIG. 7A shows waveforms of a motor current specifying signal Ir_u1 and acarrier signal fc_1 in the PWM signal generation circuit 414, FIG. 7Bshows waveforms of a motor current specifying signal Ir_u2 and a carriersignal fc_2 in a PWM signal generation circuit 415, and FIG. 7C showswaveforms of a motor current specifying signal Ir_u3 and a carriersignal fc_3 in a PWM signal generation circuit 416.

FIG. 8A is a top view of an inverter group 200, and FIG. 8B is across-sectional view thereof, taken along a line a-a′.

FIG. 9 shows waveforms of actual motor currents (only U-phases)pertaining to the embodiment when the operating frequencies ofthree-phase PWM voltages output from three-phase inverters 201, 202 and203 are 10 kHz, 20 kHz and 10 kHz, respectively.

FIG. 10 shows waveforms of actual motor currents (only U-phases)pertaining to the embodiment when the operating frequencies ofthree-phase PWM voltages output from the three-phase inverters 201, 202and 203 are 20 kHz, 20 kHz and 20 kHz, respectively.

FIG. 11 shows waveforms of actual motor currents (only U-phases)pertaining to the embodiment when the operating frequencies ofthree-phase PWM voltages output from the three-phase inverters 201, 202and 203 are 10 kHz, 10 kHz and 10 kHz, respectively.

FIG. 12 shows close-ups of the waveforms in FIG. 9, FIG. 10 and FIG. 11for comparison.

FIG. 13 shows the levels of vibration caused by rotation of thesynchronous motor, per frequency component of the vibration.

FIGS. 14A-14C show positional relationships between a stator and a rotorof Embodiment 1 of the present invention.

FIG. 15 shows temporal change of the currents applied by the three-phaseinverters to the stator coils.

FIG. 16 shows an overall structure of a synchronous motor drive systemof a modification example pertaining to Embodiment 1.

FIG. 17 shows the details of the structure of a PWM control unit 405.

FIG. 18 shows the details of a synchronous motor 304.

FIG. 19 shows connections of stator coils in the synchronous motor inFIG. 18.

FIGS. 20A-20C show positional relationships between a stator and a rotorof the synchronous motor 304.

FIG. 21 shows temporal change of the currents that are applied to thestator coils of the modification example of Embodiment 1.

FIG. 22 shows an overall structure of a synchronous motor drive systempertaining to Embodiment 2 of the present invention.

FIG. 23 shows the details of the structure of a PWM control unit 407.

FIG. 24 is a flowchart showing procedures for allocating carriersignals, performed by a carrier signal selection circuit 417.

FIG. 25 is a flowchart showing procedures for allocating carriersignals, pertaining to the modification example of Embodiment 1.

FIG. 26 shows an overall structure of a synchronous motor drive systempertaining to Embodiment 3 of the present invention.

FIG. 27 shows the structure of a gate control circuit pertaining to amodification example of Embodiment 3.

FIG. 28 shows an overall structure of a synchronous motor drive systempertaining to Embodiment 4 of the present invention.

FIG. 29 shows waveforms of the actual motor currents (only U-phases) inthe case where the operating frequencies of the three-phase PWM voltagesoutput from the three-phase inverters 201, 202 and 203 are 20 kHz, 10kHz and 20 kHz, respectively.

FIG. 30 shows waveforms of the actual motor currents (only U-phases) inthe case where the operating frequencies of the three-phase PWM voltagesoutput from the three-phase inverters 201, 202 and 203 are 10 kHz, 20kHz and 20 kHz, respectively.

FIG. 31 shows waveforms of the actual motor currents (only U-phases) inthe case where the operating frequencies of the three-phase PWM voltagesoutput from the three-phase inverters 201, 202 and 203 are 20 kHz, 20kHz and 10 kHz, respectively.

FIGS. 32A, 32B and 32C show waveforms of the motor current specifyingsignals and the carrier signals in the PWM signal generation circuits414, 415 and 416, respectively, in the case where the carrier signalsfc_1 and fc_3 are out of phase with each other.

FIG. 33 shows an overall structure of a synchronous motor drive systempertaining to a modification example of the present invention.

DESCRIPTION OF EMBODIMENTS

The following explains embodiments of a synchronous motor drive systempertaining to the present invention, with reference to the drawings.

Embodiment 1 <Synchronous Motor Drive System>

FIG. 1 shows an overall structure of a synchronous motor drive systempertaining to Embodiment 1 of the present invention.

The synchronous motor drive system includes a DC power source 100, aninverter group 200, a synchronous motor 300, a control circuit 400 and acurrent detection module 500.

The DC power source 100 provides direct current to the inverter group200.

The inverter group 200 includes three-phase inverters 201, 202 and 203.Each of the three-phase inverters 201, 202 and 203 performs DC-ACconversion according to a gate control signal from the control circuit400, and provides three-phase alternate current to the synchronous motor300. The three-phase inverter 201, 202 and 203 are composed of powercircuits 207, 208 and 209 and gate drive circuits 204, 205 and 206corresponding to the power circuits, respectively. All the switchingdevices included in the three-phase inverters 201, 202 and 203 areincluded in a single module.

The synchronous motor 300 includes three-phase coils 301, 302 and 303connected to the three-phase inverter 201, 202 and 203, respectively.The synchronous motor 300 is driven to rotate by AC power provided fromthe inverter group 200.

The motor currents output from the three-phase inverters 201, 202 and203 to the three-phase coils 301, 302 and 303 are detected by currentdetectors 51, 52, 53, 54, 55, 56, 57, 58 and 59 that are included in thecurrent detection module 500. The current values detected by the currentdetectors are input to the control circuit 400 and are used for feedbackcontrol for obtaining a desired alternate current.

This concludes the description of the outline of the synchronous motordrive system pertaining to Embodiment 1.

The following explains the details of each of the componentsconstituting the synchronous motor drive system.

<Synchronous Motor 300>

The following explains the details of the synchronous motor 300, withreference to FIGS. 2-7.

FIG. 2 is a plan view of the synchronous motor 300. FIG. 3 shows thedetails of the synchronous motor in FIG. 2.

The synchronous motor 300 includes a rotor 2 and a stator 3.

The rotor 2 includes a rotor core 4 and a plurality of permanent magnets5. The permanent magnets 5 are arranged on the rotor core 4 at equalintervals along the rotation direction of the rotor 2. The synchronousmotor 300 is an interior permanent magnet synchronous motor (i.e. IPMmotor). The permanent magnets 5 are arranged inside the rotor core. Thepermanent magnets 5 form magnetic poles 6 composed of pairs of N and Spoles. The N poles and the S poles are alternately arranged around thestator 3. It is defined that the pairs of N and S poles are providedevery 2π radians in terms of electrical angle. The interval between twoadjacent magnetic poles is π radians in terms of electrical angle. Inthe present embodiment, the rotor has twenty magnetic poles. Thus, theelectrical angle is ten times the mechanical angle.

The stator 3 includes a plurality of stator teeth 7 facing the rotor 2.The stator 3 also includes a plurality of stator coils 9 individuallywound around the stator teeth 7 by concentrated winding. Every adjacentthree of the stator teeth 7 belong to a different stator teeth group 8.In the present embodiment, the teeth groups 8 are provided every 60° interms of mechanical angle, equally. That is, six stator teeth groups 8are provided in the embodiment.

The number of the magnetic poles arranged in the rotation direction ofthe rotor 2 is twenty in total. The number of the stator teeth is 18 intotal. That is, the magnetic poles and the stator teeth are displacedfrom each other and occur at a ratio of 10 to 9 per semicircle.

Assume in FIG. 2 that the anti-clockwise direction is the + direction ofthe rotation. With respect to a stator teeth group 8 a, a stator teethgroup 8 b is arranged with an offset of −60° in terms of mechanicalangle, or +2π/3 radians in terms of electrical angle. With respect tothe stator teeth group 8 a, a stator teeth group 8 c is arranged with anoffset of +60° in mechanical angle, or +4π/3 radians (i.e. −2π/3radians) in electrical angle. Thus, the stator teeth group 8 a, thestator teeth group 8 b and the stator teeth group 8 c are provided every2/3π radians in electrical angle.

In the synchronous motor pertaining to the present embodiment, thecombination of the stator teeth group 8 a, the stator teeth group 8 band the stator teeth group 8 c is arranged twice in the rotationdirection (i.e. a stator teeth group 8 a′, a stator teeth group 8 b′ anda stator teeth group 8 c′ are arranged as well).

The following describes the details of the structure of the stator teethgroup 8 a, with reference to FIG. 3. In the following description, themechanical angle between adjacent two stator coils is represented by theangle between the center lines (i.e. the dashed-dotted lines) of thestator teeth wound with the stator coils. The stator teeth group 8 a iscomposed of three adjacent stator teeth 61 a, 62 a and 63 a. The statorteeth 61 a, 62 a and 63 a are respectively wound with the stator coils81 a, 82 a, and 83 a by concentrated winding such that the windingdirections of the stator coils 81 a, 82 a, and 83 a are alternatelyopposite to each other. The stator tooth 61 a wound with the stator coil81 a is positioned with an offset of +20° in mechanical angle withrespect to the stator tooth 62 a wound with the stator coil 82 a. Thatis, the stator coil 82 a is arranged with an additional offset of +π/9radians besides the offset of π radians in electrical angle (i.e. 18° inmechanical angle) between the magnetic poles. Similarly, the stator coil83 a is arranged with an offset of −20° in mechanical angle with respectto the stator coil 82 a. That is, the stator coil 83 a is arranged withan additional offset of −π/9 radians besides the offset of π radians inelectrical angle between the magnetic poles. Here, note that the statorteeth are arranged in the rotation direction at equal intervals of360/18=20°. On the other hand, twenty magnetic poles of the rotor areprovided in the rotation direction at equal intervals of 360/20=18°.

This also applies to the other two stator teeth groups 8 b and 8 c shownin FIG. 2. That is, similarly to the stator teeth group 8 a, three coilsof each stator teeth group are arranged with an additional offset of+π/9 radians or −π/9 radians in electrical angle besides the offset of πradians in electrical angle, which is the interval between the magneticpoles.

FIG. 4 is a drawing for explaining connections of the stator coils inthe synchronous motor in FIG. 2.

The signs “a”, “b” and “c” correspond to the coils contained in thestator teeth groups 8 a, 8 b and 8 c, respectively.

Coil terminals 31 a, 32 a and 33 a of the three stator coils 81 a, 82 aand 83 a belonging to the stator teeth group 8 a each extend outside,and are connected to the U-phase connection terminals of the three-phaseinverters 201, 202 and 203, respectively. Similarly, three coilterminals 31 b, 32 b and 33 b belonging to the stator teeth group 8 band three coil terminals 31 c, 32 c and 33 c belonging to the statorteeth group 8 c each extend outside, and are connected to the V-phaseand W-phase connection terminals of the three-phase inverters 201, 202and 203, respectively.

Additionally, among the stator coil terminals in different stator teethgroups 8 a, 8 b and 8 c, coil terminals having a phrase difference of2π/3 radians are connected to a common neutral point. That is, thestator coil terminals 34 a, 34 b and 34 c are connected to a firstneutral point, the stator coil terminals 35 a, 35 b and 35 c areconnected to a second neutral point, and the stator coil terminals 36 a,36 b and 36 c are connected to a third neutral point. In this example,the first, the second and the third neutral points are electricallydisconnected. However, any two or all of the neutral points may beelectrically connected with each other.

The present embodiment includes two pairs of stator teeth groups 8 a,two pairs of stator teeth groups 8 b and two pairs of stator teethgroups 8 c. Stator teeth groups with the same suffix (i.e. a, b or c)have the same positional relationship (i.e. the same electrical angle)with respect to the magnetic poles of the rotor. Thus, three adjacentgroups of the six stator teeth groups may connect to a single neutralpoint, or three alternately-arranged groups of the six stator teethgroups may connect to a single neutral point. Alternatively, all the sixstator teeth groups may connect to a single neutral point.

With such connections, the stator coils 81 a, 81 b, 81 c, 81 a′, 81 b′and 81 c′, whose respective coil terminals are connected to thethree-phase inverter 201, compose the three-phase coil 301 shown inFIG. 1. Similarly, the stator coils 82 a, 82 b, 82 c, 82 a′, 82 b′ and82 c′, whose respective coil terminals are connected to the three-phaseinverter 202, compose the three-phase coil 302 shown in FIG. 1; and thestator coils 83 a, 83 b, 83 c, 83 a′, 83 b′ and 83 c′, whose respectivecoil terminals are connected to the three-phase inverter 203, composethe three-phase coil 303 shown in FIG. 1

This concludes the description of the structure of the synchronous motorincluded in the synchronous motor drive system pertaining to Embodiment1 of the present invention. The eighteen stator teeth are arranged withintervals that are different from the intervals of the magnetic poles ofthe rotor. Every three stator teeth constitutes a stator teeth group.Every three stator coils of each stator teeth group are separatelyconnected to an individual external terminal.

Here, note that the term “separately” refers to the relationship amongthe stator coils contained in a single stator teeth group, and not tothe relationship among the stator coils contained in different statorteeth groups. Thus, stator coils contained in different stator teethgroups may be connected to the same external terminal, if conditionspermit. For example, the stator coil 81 a contained in the stator teethgroup 8 a and the stator coil 81 a′ contained in the stator teeth group8 a′ may be connected to the same external terminal, because currents inthe same phase are supplied thereto. Of course, they may be connected todifferent external terminals separately.

<Control Circuit 400>

The following describes the details of the control circuit 400.

As shown in FIG. 1, the control circuit 400 includes a PWM control unit401, a current detection unit 402 and a position estimating unit 403.The control circuit 400 controls the operations of the three-phaseinverter 201, 202 and 203 by outputting gate control signals GU_u andGU_d thereto.

The PWM control unit 401 outputs gate control signals to the inverters,based on motor current instruction signals Ir_u, Ir_v and Ir_w output bythe current detection unit 402.

The current detection unit 402 receives a torque specifying signal Isand a rotation speed specifying signal or input from the outside. Thetorque specifying signal and the rotation speed specifying signalspecifying the desired torque and rotation speed for driving thesynchronous motor 300, respectively. The current detection unit 402determines, for each of the inverter 201, 202 and 203, a current phaseangle β and a current amount Ia according to the torque specifyingsignal Is and the rotation speed specifying signal ωr that have beenreceived. The current detection unit 402 outputs the motor currentspecifying signals Ir_u, Ir_(—v and Ir)_w to the PWM control unit 401while monitoring the positions of the magnetic poles of the rotor of thesynchronous motor and the current values of the power-supply wirings.

The position estimating unit 403 receives at least one three-phasealternate current detection signal detected by the current detectionmodule 500, and calculates the inductance of the coils from the currentchange ratio per switching of the power circuit. The position estimatingunit 403 estimates a rotor magnetic pole position θ of the synchronousmotor 300 from the inductance, and outputs the rotor magnetic poleposition θ to the current detection unit 402.

The following describes the structure and the operations of the PWMcontrol unit 401, with reference to FIG. 5.

The PWM control unit 401 includes carrier signal generation circuits411, 412 and 413 and PWM signal generation circuits 414, 415 and 416.The carrier signal generation circuit 411 outputs a carrier signal fc_1,which is a 10 kHz triangle wave, to the PWM signal generation circuit414. The carrier signal generation circuit 412 outputs a carrier signalfc_2, which is a 20 kHz triangle wave, to the PWM signal generationcircuit 415. The carrier signal generation circuit 413 outputs a carriersignal fc_3, which is a 10 kHz triangle wave, to the PWM signalgeneration circuit 416.

Each of the PWM signal generation circuit 414, 415 and 416 receives acarrier signal and motor current specifying signals Ir_u, Ir_v and Ir_w,and outputs a gate control signal based on the input signals.

Here, FIG. 6 shows an example pattern of a gate control signal generatedby the PWM signal generation circuit 414. The PWM signal generationcircuit 414 receives the carrier signal fc_1 and the motor currentspecifying signal Ir_u1 output from the current detection unit 402, andcompares these two signals. When the motor current specifying signalIr_u1 is greater than the carrier signal fc_1, the PWM signal generationcircuit 414 outputs a gate control signal GU_u1 (i.e. High level in thedrawing) that turns on the upper arm corresponding to the motor currentspecifying signal Ir_u1. When the motor current specifying signal Ir_u1is less than the carrier signal fc_1, the PWM signal generation circuit414 outputs a gate control signal GU_u1 (i.e. Low level in the drawing)that turns off the upper arm corresponding to the motor currentspecifying signal Ir_u1. The logic level of the gate control signalGU_d1 for the lower arm is reversed from the gate control signal GU_u1for the upper arm. Here, the gate control signals GU_u and GU_d aresupplied with a pause for a micro time, in which both signals are at Lowlevel. This is for preventing a short circuit between the upper andlower arms.

In the drawing, generation of a gate control signal is explained basedon the motor current specifying signal Ir_u1 for one phase of thethree-phase alternate current. However, note that the PWM signalgeneration circuit 414 generates gate control signals for the otherphases, namely V and W phases, based on the motor current specifyingsignals Ir_v1 and Ir_w1, which are out of phase with the motor currentspecifying signal Ir_u1 by 120° and 240°, respectively.

As shown in FIGS. 7A-7C, each of the PWM signal generation circuits 415and 416 receives a motor current specifying signal and a carrier signalin the same manner as in the PWM signal generation circuit 414, andgenerates a gate control signal based on the received signals.

FIG. 7A shows the waveforms of the motor current specifying signal Ir_u1and the carrier signal fc_1 in the PWM signal generation circuit 414.FIG. 7B shows the waveforms of the motor current specifying signal Ir_u2and the carrier signal fc_2 in the PWM signal generation circuit 415.FIG. 7C shows the waveforms of the motor current specifying signal Ir_u3and the carrier signal fc_3 in the PWM signal generation circuit 416.

Among the gate control signals generated based on the input signalshaving such waveforms, the gate control signal generated by the PWMsignal generation circuit 414 by using the carrier signal fc_1 as a 10kHz triangle wave is input to the gate drive circuit 204 of thethree-phase inverter 201. The gate control signal generated by the PWMsignal generation circuit 415 by using the carrier signal fc_2 as a 20kHz triangle wave is input to the gate drive circuit 205 of thethree-phase inverter 202. The gate control signal generated by the PWMsignal generation circuit 416 by using the carrier signal fc_3 as a 10kHz triangle wave is input to the gate drive circuit 206 of thethree-phase inverter 203.

As a result, the operating frequency of the three-phase PWM voltagesoutput from the three-phase inverters 201, 202 and 203 in the presentembodiment will be 10 kHz, 20 kHz and 10 kHz, respectively. Thethree-phase PWM voltages output from the three-phase inverters 201, 202and 203 are input to the three-phase coils 301, 302 and 303,respectively. Regarding the stator coils shown in FIG. 3, thethree-phase PWM voltages input to the stator coils 81 a and 83 a arewith 10 kHz, and the three-phase PWM voltage input to the stator coil 82a between the stator coils 81 a and 83 a are with 20 kHz.

This concludes the description of the details of the control circuit400.

<Inverter Group 200>

The following describes the details of the inverter group 200. FIG. 8Ais a top view of the inverter group 200, and FIG. 8B is across-sectional view thereof, taken along a line a-a′. As shown in thedrawings, the inverter group 200 constitutes a single module, in whichthe three-phase inverters 201, 202 and 203 are arranged next to eachother on an insulating substrate 240 and are sealed with a resin mold250 such as an epoxy resin.

Here, the three-phase inverter 201 and the three-phase inverter 203operate at the operating frequency of 10 kHz, and the three-phaseinverter 202 operates at the operating frequency of 20 kHz. Thus thethree-phase inverter 202 generates a larger amount of heat than theother three-phase inverters. In view of this, the three-phase inverter202 is structured from a switching device with use of a wide-bandgapsemiconductor such as silicon carbide and gallium nitride with excellentthermal resistance, which has a wider bandgap than Si semiconductors,whereas the three-phase inverter 201 and the three-phase inverter 203are structured from switching devices with use of a cheap Sisemiconductor.

Thus, in the module of the inverter group 200, the heat gradient of thethree-phase inverters 201, 202 and 203 is substantially symmetry withrespect to the three-phase inverter 202. This results in highreliability. Also, with such a structure, it is possible to provide amodule at a lower cost than structuring the module only with awide-bandgap semiconductor.

Furthermore, in the case of applying an IPM (Intelligent Power Module),it is possible to provide a module with high reliability by providing adriving circuit for controlling a three-phase inverter, only on thethree-phase inverter with a lower operating frequency, because thisreduces the increase in the temperature of the atmosphere around thedriving circuit.

This concludes the description of the details of the inverter group 200.

Next, implementation of the synchronous motor drive system pertaining tothe present invention and its effect are described with reference toFIGS. 9-12.

FIG. 9 shows the waveforms of the actual motor currents (only U-phases)pertaining to the embodiment when the operating frequencies of thethree-phase PWM voltages output from the three-phase inverters 201, 202and 203 are 10 kHz, 20 kHz and 10 kHz, respectively. FIG. 10 is similarto the embodiment. However, FIG. 10 shows the waveforms of the actualmotor currents (only U-phases) pertaining to the embodiment when theoperating frequencies of the three-phase PWM voltages output from thethree-phase inverters 201, 202 and 203 are 20 kHz, 20 kHz and 20 kHz,respectively. Similarly, FIG. 11 shows the waveforms of the actual motorcurrents (only U-phases) pertaining to the embodiment when the operatingfrequencies of the three-phase PWM voltages output from the three-phaseinverters 201, 202 and 203 are 10 kHz, 10 kHz and 10 kHz, respectively.FIG. 12 shows close-ups of the waveforms in FIG. 9, FIG. 10 and FIG. 11for comparison.

As seen from FIG. 10 and the second stage of FIG. 12, the currentripples, or the waveform distortions of the actual motor currentsapplied to the three-phase coils 301, 302 and 303 are reduced when allthe operating frequencies are 20 kHz. On the other hand, as seen fromFIG. 11 and the third stage of FIG. 12, the current ripples, or thewaveform distortions of the actual motor currents applied to thethree-phase coils 301, 302 and 303 are large when all the operatingfrequencies are 10 kHz. Specifically, as shown in FIG. 12, the waveformdistortion in the second stage where all the operating frequencies 20kHz is approximately ½ of the waveform distortion in the third stagewhere all the operating frequencies are 10 kHz. Waveform distortions ofan actual motor current cause serious problems in driving a motor,namely, noise and vibration of the motor.

However, regarding the actual motor current waveform of the embodimentshown in FIG. 9, the ripples of the actual motor current I_u2 of thethree-phase coil 302, to which a 20 kHz three-phase PWM voltage isinput, are reduced as is in FIG. 10. Furthermore, the ripples of theactual motor currents I_u2 and I_u3 of the three-phase coils 301 and303, to which 20 kHz three-phase PWM voltages are input, aresignificantly reduced when compared with those in FIG. 11. Specifically,as shown in FIG. 12, the distortion of the actual motor current waveformin the first stage, of the three-phase coil to which a 10 kHzthree-phase PWM voltage is input in the present embodiment, is within arange from approximately ½ to approximately ⅗ of that in the third stagewhere all the operating frequencies are 10 kHz.

It can be assumed that such an effect is due to the following reasons:in implementation of the present invention, by setting the operatingfrequencies of the three-phase PWM voltages to be input to thethree-phase coils 301, 302 and 303 at 10 kHz, 20 kHz and 10 kHz, thechange of magnetic flux caused by the current applied to the three-phasecoil 302 with the operating frequency of 20 kHz suppresses the changesof magnetic flux caused by the currents applied to the three-phase coils301 and 303 with the operating frequency of 10 kHz, and thissignificantly reduces the waveform distortion of the actual motorcurrent.

Also, vibration in the synchronous motor is reduced by implementing thepresent invention. FIG. 13 shows the levels of vibration caused byrotation of the synchronous motor, per frequency component of thevibration. The first stage shows vibration in the embodiment when thethree-phase inverters 201, 202 and 203 are operated with carrier signalswith the frequencies of 10 kHz, 20 kHz and 10 kHz, respectively. Thesecond stage shows the case where all the three-phase inverters areoperated with a carrier signal with the frequency of 10 kHz. The thirdstage shows the case where all the three-phase inverters are operatedwith a carrier signal with the frequency of 20 kHz.

In the frequency bands shaded in the drawing, the vibration is reduce toa low level under the conditions of the present embodiment than underother conditions. Thus, the effect of reducing the vibration is achievedin the whole synchronous motor drive system with the resonance frequencywithin such frequency bands.

Specifically, at the peak p1, the vibration level in the second stage,where all the three-phase inverters are operated with a carrier signalof 10 kHz, is approximately −70 dB. On the other hand, the vibrationlevel under the conditions of the present invention is reduced toapproximately −80 dB as shown in the second stage. Similarly, at thepeak p2, the vibration level in the second stage, where all thethree-phase inverters are operated with a carrier signal of 10 kHz, isapproximately −77 dB. On the other hand, the vibration level under theconditions of the present invention is reduced to approximately −80 dBas shown in the first stage. Also, at the peak p3, the vibration levelin the second stage, where all the three-phase inverters are operatedwith a carrier signal of 10 kHz, is approximately −70 dB. On the otherhand, the vibration level under the conditions of the present inventionis reduced to approximately −90 dB as shown in the first stage.

In the present embodiment as described above, by operating some of thethree-phase inverters based on a gate control signal generated with acarrier signal of 10 kHz, the switching loss is reduced to be lower thanthe case where all the three-phase inverters are operated based on gatecontrol signals generated with a carrier signal of 20 kHz. Meanwhile,the waveform distortion of the motor current due to the ripples isreduced to be a lower level than the case where the all the three-phaseinverters are operated based on gate control signals generated with thefrequency of 10 kHz.

In this way, implementation of the present invention provides a motordrive system that solves important problems such as switching loss ofthe three-phase inverters and the EMC, caused along with the increase ofthe operating frequency, and realizes low noise and low vibration in themotor drive.

<Modification 1 of Embodiment 1>

In the synchronous motor drive system pertaining to Embodiment 1 of thepresent invention, it is preferable that currents that are out of phasewith each other are applied to the coil terminals of the synchronousmotor. The following explains an example method for applying a currentto the synchronous motor 300 to drive it to rotate. The structure of thesynchronous motor 300 is shown in FIG. 2 and FIG. 3.

FIGS. 14A-14C show positional relationships between the stator and therotor of Embodiment 1 of the present invention. FIG. 14A-14C show thepositional relationship in the case where the rotor is rotatedanti-clockwise by 2° in mechanical angle (i.e. π/9 radians in electricalangle) at a time. FIG. 15 shows the temporal change of the currentsapplied by the three-phase inverters to the stator coils. The timepoints (a), (b) and (c) in FIG. 15 correspond to the positionalrelationships shown in FIG. 14A, FIG. 14B and FIG. 14C, respectively.

In FIG. 2 and FIG. 3, points between magnetic poles of the rotor areindicated by the signs 10 and 11. Each of the points 10 and 11 betweenthe magnetic poles of the rotor is a magnetic neutral point between an Nmagnetic pole and an S magnetic pole each generated with a permanentmagnet located in the rotor. Here, it is assumed that each neutral pointis located between the magnets, in terms of the physical locations aswell. The point 10 shows the magnetic neutral point at which themagnetic property changes from N to S anti-clockwise, and the point 11shows the magnetic neutral point at which the magnetic property changesfrom S to N anti-clockwise. Note that the point 11′ is at the sameposition as the point 11 in terms of electrical angle, but at differentpositions in terms of mechanical angle.

In FIG. 14A, as shown as the dashed-dotted line, the center point of thestator tooth 63 a and the point 11 between the rotor magnetic poles arein line. If this is the case, the magnet torque, which is the torque ofthe permanent magnet, reaches its maximum when the phase of the currentsupplied is controlled such that the current that flows through thestator coil 83 a connected to the inverter 203 is controlled so as to bemaximized. As explained in FIG. 3, the angle between adjacent magneticpoles (i.e. 18°) and the angle between adjacent stator teeth (20°) aredifferent. Thus, when the center point of the stator tooth 63 a and thepoint 11 between the rotor magnetic poles are in line, the center pointof the stator tooth 62 a and the point 10 between the rotor magneticpoles, and the center point of the stator tooth 61 a and the point 11′between the rotor magnetic poles are not in line.

In FIG. 14B, the rotor is rotated anti-clockwise from that shown in FIG.14A by 2° in mechanical angle (i.e. π/9 radians in electrical angle). Asshown as the dashed-dotted line, the center point of the stator tooth 62a and the point 10 between the rotor magnetic poles are in line. If thisis the case, the magnet torque, which is the torque of the permanentmagnet, reaches its maximum when the phase of the current supplied iscontrolled such that the current that flows through the stator coil 82 aconnected to the inverter 202 is controlled so as to be maximized. Here,the center point of the stator tooth 63 a and the point 11 between therotor magnetic poles, and the center point of the stator tooth 61 a andthe point 11′ between the rotor magnetic poles are not in line. In FIG.14C, the rotor is rotated anti-clockwise from that shown in FIG. 14B by2° in mechanical angle (i.e. π/9 radians in electrical angle). As shownas the dashed-dotted line, the center point of the stator tooth 61 a andthe point 11′ between the rotor magnetic poles are in line. If this isthe case, the magnet torque, which is the torque of the permanentmagnet, reaches its maximum when the phase of the current supplied iscontrolled such that the current that flows through the stator coil 81 aconnected to the inverter 201 is controlled so as to be maximized. Here,the center point of the stator tooth 63 a and the point 11 between therotor magnetic poles, and the center point of the stator tooth 62 a andthe point 10 between the rotor magnetic poles are not in line.

The phase of the current to be applied is controlled such that thecurrents flowing through the stator coils 81 a, 82 a and 83 a are attheir maximum when the positional relationships are as shown in FIGS.14A, 14B and 14C, that is, when each of the respective center points ofthe stator teeth 61 a, 62 a and 63 a are in line with a point betweenrotor magnetic poles. This maximizes the magnet torque for each of thestator teeth, thereby increasing the total torque.

In FIG. 15, the vertical axis shows the currents applied by thethree-phase inverters 201, 202 and 203 to the coil terminals 31 a, 32 aand 33 a (i.e. to the stator coils 81 a, 82 a and 83 a), and thehorizontal axis shows the time. As shown in FIG. 15, the current appliedto the coil terminal 33 a is set ahead of the current applied to thecoil terminal 32 a by π/9 radians, and the current applied to the coilterminal 31 a is delayed from the current applied to the coil terminal32 a by π/9 radians.

As explained above, in the synchronous motor 300, the intervals of therotor magnetic poles are 18° in mechanical angle (i.e. π radians inelectrical angle) whereas the intervals of the three stator teeth ineach stator teeth group are 20° in mechanical angle, instead of 18°.With such a mechanical phase difference, the synchronous motor 300reduces the cogging torque, which is torque ripples while no electricalpower is being supplied.

In the synchronous motor 300, each of the stator teeth in each statorteeth group is located with a phase difference of π/9 radians withrespect to π radians in electrical angle. By supplying currents to thestator coils wound around the stator teeth such that the current flowingto each stator coil has a phase difference of π/9 radians, each statortooth yields the same amount of torque. As a result, the torque ripplehaving a fundamental period of Π/3 radians is cancelled, and the torqueyielded by each stator tooth is maximized, whereby the overall torque isincreased.

Note in FIG. 14 that only the magnet torque generated by the permanentmagnets is taken into consideration. Thus, the phases of currents areadjusted so as to maximize the current flowing to a stator coil when thecenter point of the stator tooth and the point between magnetic poles ofthe rotor are in line. However, the synchronous motor pertaining toEmbodiment 1 is what is called an interior permanent magnet synchronousmotor that has permanent magnets arranged inside the rotor core. Thesynchronous motor utilizes reluctance torque resulting from a differencein magnetoresistance, along with the magnet torque generated by themagnets. In order to maximize the torque by utilizing both the magnettorque and the reluctance torque, it is in some cases effective toadvance the phases of the currents so as not to maximize the currentflowing to a stator coil when the center of the stator tooth and thepoint between magnetic poles of the rotor are aligned and facing eachother.

<Modification 2 of Embodiment 1>

A description is made below of a modification in which the presentinvention is applied to a synchronous motor drive system including twoinverters. FIG. 16 shows an overall structure of a synchronous motordrive system pertaining to this modification.

In the synchronous motor drive system pertaining to this modification,the inverter group 200, the synchronous motor 300, the control circuit400 and the current detection module 500 included in the system shown inFIG. 1 are replaced with a inverter group 210, a synchronous motor 304,a control circuit 404 and a current detection module 501, respectively.The following explains the differences form the structure shown in FIG.1.

The inverter group 210 has the same structure as the inverter group 200except that the three-phase inverter 203 is removed.

The synchronous motor 304 includes two three-phase coils 305 and 306.

The control circuit 404 has the same structure as the control circuit400 shown in FIG. 1 except that the PWM control unit 401 is replacedwith a PWM control unit 405. FIG. 17 shows the details of the structureof the PWM control unit 405. The PWM control unit 405 has the samestructure as the PWM control unit 401 shown in FIG. 5 except that thecarrier signal generation circuit 413 and the PWM signal generationcircuit 416 are removed. With this structure, a gate control signalusing a 10 kHz carrier signal is output from the PWM control unit 405 tothe gate drive circuit 204 of the three-phase inverter 201, and a gatecontrol signal using a 20 kHz carrier signal is output to the gate drivecircuit 205 of the three-phase inverter 202.

This concludes the description of the outline of the synchronous motordrive system pertaining to this modification.

Next, the details of the synchronous motor 304 are described. Thesynchronous motor 304 has a structure similar to the structure ofsynchronous motor 300 shown in FIG. 2. However, the manner in which astator coil is wound around each stator tooth is different from thesynchronous motor 300.

FIG. 18 shows the details of the synchronous motor 304. The followingdescribes the details of the structure of the stator teeth group 8 a,with reference to FIG. 18. In the following description, the mechanicalangle between adjacent stator teeth is represented by the angle betweenthe center lines (i.e. the dashed-dotted lines) of the correspondingstator teeth around which the stator coils are wound. The stator teethgroup 8 a is composed of three adjacent stator teeth 71 a, 72 a and 73a.

The stator tooth 71 a is positioned with an offset of +20° in mechanicalangle with respect to the stator tooth 72 a. That is, the stator coil 83a is arranged with an additional offset of +π/9 radians besides theoffset of π radians in electrical angle (i.e. 18° in mechanical angle)between the magnetic poles. Similarly, the stator tooth 73 a ispositioned with an offset of −20° in mechanical angle with respect tothe stator tooth 72 a. That is, the stator coil 73 a is arranged with anadditional offset of −π/9 radians besides the offset of π radians inelectrical angle between the magnetic poles. Here, note that the statorteeth are arranged in the rotation direction at equal intervals of360/18=20°. On the other hand, twenty magnetic poles of the rotor aredistributed in the rotation direction at equal intervals of 360/20=18°.

Around the stator tooth 71 a, part of the stator coil 91 a (the numberof turns is N1) is wound. Around the stator tooth 73 a, part of thestator coil 92 a (the number of turns is N2) is wound. Around the statortooth 72 a, the rest of the stator coil 91 a (the number of turns isN21) and the rest of the stator coil 92 a (the number of turns is N22)are wound.

In the stator coil 91 a, the part wound around the stator tooth 71 a andthe part wound around the stator tooth 72 a generate magnetic fieldshaving reversed polarities. Similarly, in the stator coil 92 a, the partwound around the stator tooth 72 a and the part wound around the statortooth 73 a generate magnetic fields having reversed polarities.Furthermore, when currents in the same phase are supplied to the statorcoils 91 a and 92 a, the parts wound around the stator tooth 72 agenerate magnetic fields having the same polarity.

Regarding the stator coils 91 a and 92 a, the number of turns satisfiesthe following equations:

N1=N2; and

N21=N22=(N1)/{2 cos(π/9)}

These equations equalize the maximum values of the magnetic fluxgenerated by the stator teeth 71 a, 72 a and 73 a. Note that althoughequal signs (i.e. “=”) are used above, it is often difficult to achieveperfect matching. The equal signs above are assumed to represent thecases where, for example, the right side is a decimal that can be roundoff to the integer on the left side. Moreover, a difference that isnegligible as a design error may be included in the cases.

The two stator teeth groups 8 b and 8 c shown in FIG. 18, both adjacentto the stator teeth group 8 a, have the same structure as the statorteeth group 8 a shown in FIG. 18.

FIG. 19 is a drawing for explaining connections of the stator coils inthe synchronous motor in FIG. 18.

The signs “a”, “b” and “c” correspond to the coils contained in thestator teeth groups 8 a, 8 b and 8 c, respectively.

Coil terminals 21 a and 23 a of the two stator coils 91 a and 92 abelonging to the stator teeth group 8 a separately extend outside. Thecoil terminal 21 a is connected to the U-phase connection terminal ofthe three-phase inverters 201, and the coil terminal 23 a is connectedto the U-phase connection terminal of the three-phase inverter 202.Similarly, two coil terminals 21 b and 23 b belonging to the statorteeth group 8 b and two coil terminals 21 c and 23 c belonging to thestator teeth group 8 c separately extend outside, and are connected tothe V-phase and W-phase connection terminals of the three-phaseinverters 201 and 202 respectively.

Additionally, among the stator coil terminals in different stator teethgroups 8 a, 8 b and 8 c, coil terminals having a phrase difference of2π/3 radians are connected to a common neutral point. That is, thestator coil terminals 22 a, 22 b and 22 c are connected to a firstneutral point, and the stator coil terminals 24 a, 24 b and 24 c areconnected to a second neutral point. In this example, the first and thesecond neutral points are electrically disconnected. However, theseneutral points may be electrically connected with each other.

The present embodiment includes two pairs of stator teeth groups 8 a,two pairs of stator teeth groups 8 b and two pairs of stator teethgroups 8 c. Stator teeth groups with the same suffix (i.e. a, b or c)have the same positional relationship (i.e. the same electrical angle)with respect to the magnetic poles of the rotor. Thus, three adjacentpairs of the six pairs of stator teeth groups may connect to a singleneutral point, or three alternately-arranged pairs of the six pairs ofstator teeth groups may connect to a single neutral point.Alternatively, all the six pairs of stator teeth groups may connect to asingle neutral point.

With such connections, the stator coils 91 a, 91 b, 91 c, 91 a′, 91 b′and 91 c′, whose respective coil terminals are connected to thethree-phase inverter 201, compose the three-phase coil 305 shown in FIG.16. Similarly, the stator coils 92 a, 92 b, 92 c, 92 a′, 92 b′ and 92c′, whose respective coil terminals are connected to the three-phaseinverter 202, compose the three-phase coil 306 shown in FIG. 16.

This concludes the description of the structure of the synchronous motor304. The eighteen stator teeth are arranged with intervals that aredifferent from the intervals of the magnetic poles of the rotor. Everythree stator teeth constitutes a stator teeth group. Every two statorcoils of each stator teeth group are separately connected to anindividual external terminal.

Here, note that the term “separately” refers to the relationship amongthe stator coils contained in a single stator teeth group, and not tothe relationship among the stator coils contained in different statorteeth groups. Thus, stator coils contained in different stator teethgroups may be connected to the same external terminal, if conditionspermit. For example, the stator coil 91 a contained in the stator teethgroup 8 a and the stator coil 91 a′ contained in the stator teeth group8 a′ may be connected to the same external terminal, because currents inthe same phase is supplied thereto. Of course, they may be connected todifferent external terminals separately.

The synchronous motor drive system pertaining to this modificationincludes a drive apparatus for supplying currents in different phases toa plurality of coil terminals of the synchronous motor. The followingexplains the drive apparatus and methods of applying power.

FIGS. 20A-20C show positional relationships between the stator and therotor of this modification of the present invention. FIG. 20A-20C showthe positional relationship in the case where the rotor is rotatedanti-clockwise by 2° in mechanical angle (i.e. π/9 radians in electricalangle) at a time. FIG. 21 shows the temporal change of the currentsapplied by to the stator coils pertaining to this modification. The timepoints (a), (b) and (c) in FIG. 21 correspond to the positionalrelationships shown in FIG. 20A, FIG. 20B and FIG. 20C, respectively.

In FIGS. 20A-20C, each of the points 10 and 11 between the magneticpoles of the rotor is a magnetic neutral point between an N magneticpole and an S magnetic pole each generated with a permanent magnetlocated in the rotor. Here, it is assumed that each neutral point islocated between the magnets, in terms of the physical locations as well.The point 10 shows the magnetic neutral point at which the magneticproperty changes from N to S anti-clockwise, and the point 11 shows themagnetic neutral point at which the magnetic property changes from S toN anti-clockwise. Note that the point 11′ is at the same position as thepoint 11 in terms of electrical angle, but at different positions interms of mechanical angle.

In FIG. 20A, as shown as the dashed-dotted line, the center point of thestator tooth 73 a and the point 11 between the rotor magnetic poles arein line. If this is the case, the magnet torque reaches its maximum whenthe phase of the current supplied is controlled such that the currentthat flows through the stator coil 93 a is controlled so as to bemaximized. As explained in FIG. 18, the angle between adjacent magneticpoles (i.e.) 18° and the angle between adjacent stator teeth (20°) aredifferent. Thus, when the center point of the stator tooth 73 a and thepoint 11 between the rotor magnetic poles are in line, the center pointof the stator tooth 72 a and the point 10 between the rotor magneticpoles, and the center point of the stator tooth 71 a and the point 11′between the rotor magnetic poles are not in line.

In FIG. 20B, the rotor is rotated anti-clockwise from that shown in FIG.20A by 2° in mechanical angle (i.e. π/9 radians in electrical angle). Asshown as the dashed-dotted line, the center point of the stator tooth 72a and the point 10 between the rotor magnetic poles are in line. Here,the center point of the stator tooth 73 a and the point 11 between therotor magnetic poles, and the center point of the stator tooth 71 a andthe point 11′ between the rotor magnetic poles are not in line.

In FIG. 20C, the rotor is rotated anti-clockwise from that shown in FIG.20B by 2° in mechanical angle (i.e. π/9 radians in electrical angle). Asshown as the dashed-dotted line, the center point of the stator tooth 71a and the point 11′ between the rotor magnetic poles are in line. Ifthis is the case, the magnet torque reaches its maximum when the phaseof the current supplied is controlled such that the current that flowsthrough the stator coil 91 a is controlled so as to be maximized. Here,the center point of the stator tooth 73 a and the point 11 between therotor magnetic poles, and the center point of the stator tooth 72 a andthe point 10 between the rotor magnetic poles are not in line.

The phase of the current to be applied is controlled such that thecurrents flowing through the stator coils 91 a and 92 a are at theirmaximum when the positional relationships are as shown in FIGS. 20A and20C, that is, when each of the respective center points of the statorteeth 71 a and 73 a are in line with a point between rotor magneticpoles. As a result, the current flowing through the stator coil 92 a isat its maximum when the positional relationship is as shown in FIG. 20A,that is, when the center point of the stator tooth 73 a is in line withthe point 11 between rotor magnetic poles. This maximizes the magnettorque to be generated by the stator tooth 73 a. Also, the compositionof the vectors of the currents flowing through the stator coils 91 a and92 a is at its maximum when the positional relationship is as shown inFIG. 20B, that is, when the center point of the stator tooth 72 a is inline with the point 10 between rotor magnetic poles. This maximizes themagnet torque to be generated by the stator tooth 72 a. Similarly, thecurrent flowing through the stator coil 91 a is at its maximum when thepositional relationship is as shown in FIG. 20C, that is, when thecenter point of the stator tooth 71 a is in line with the point 11′between rotor magnetic poles. This maximizes the magnet torque to begenerated by the stator tooth 71 a. This maximizes the magnet torque foreach of the stator teeth, thereby increasing the total torque.

In FIG. 21, the vertical axis shows the currents applied by thethree-phase inverters 201 and 202 to the coil terminals 21 a and 23 a(i.e. to the stator coils 91 a and 92 a), and the horizontal axis showsthe time. As shown in FIG. 21, the current applied to the coil terminal23 a is set ahead of the current applied to the coil terminal 21 a by2π/9 radians.

The relationship between the positional relationships of the statorcoils and the currents to be applied to the stator coils is as follows.

With respect to the stator tooth 72 a, the stator tooth 73 a is arrangedwith an additional offset of −π/9 radians to the offset of π radians, interms of electrical angle. With respect to the stator tooth 72 a, thestator tooth 71 a is arranged with an additional offset of +π/9 radiansto the offset of π radians, in terms of electrical angle. With suchpositional relationships, the current applied to the stator coil 93 a isset ahead of the current applied to the coil 91 a by 2π/9 radians.

As explained above, in the synchronous motor drive system pertaining tothis modification, the PWM control is performed by using a lowerfrequency carrier signal in one of the two three-phase inverters thanthe other. As a result, it can be expected that, while the switchingloss in the entire inverter group 210 is reduced, the increase of thewaveform distortion of the actual motor current is suppressed by thefollowing action. That is, the changes of magnetic flux caused by thecurrent applied to the three-phase coil 305 with the operating frequencyof 20 kHz suppresses the changes of magnetic flux caused by the currentapplied to the three-phase coil 306 with the operating frequency of 10kHz.

Embodiment 2

FIG. 22 shows an overall structure of a synchronous motor drive systempertaining to Embodiment 2 of the present invention. The synchronousmotor drive system shown in FIG. 22 has the same structure as thesynchronous motor drive system shown in FIG. 1 except that the invertergroup 200 and the control circuit 400 are replaced with an invertergroup 220 and a control circuit 406. The following explains thedifferences from the synchronous motor drive system pertaining toEmbodiment 1.

Three-phase inverters 221, 222 and 223 included in the inverter group220 are different from the three-phase inverters 201, 202 and 203pertaining to Embodiment 1 in that they have temperature sensors 61, 62and 63, respectively.

The temperature sensors 61, 62 and 63 cyclically measure thetemperatures of switching devices included in power circuits 227, 228and 229 of the three-phase inverters, respectively. The temperaturesensors 61, 62 and 63 output temperature detection signals T1, T2 and T3corresponding to the power circuits 227, 228 and 229 to the controlcircuit 406, respectively.

The control circuit 406 has the same structure as the control circuit400 shown in FIG. 1 except that the PWM control unit 401 is replacedwith a PWM control unit 407. FIG. 23 shows the details of the structureof the PWM control unit 407. The PWM control unit 405 has the samestructure as the PWM control unit 401 except that a carrier signalselection circuit 417 is added thereto.

The carrier signal selection circuit 417 allocates the carrier signalsfc_1, fc_2 and fc_3 to the PWM signal generation circuits 414, 415 and416 based on the temperature detection signals T1, T2 and T3 input fromthe temperature sensors 61, 62 and 63. Here, the carrier signalselection circuit 417 determines the allocation of the carrier signalsto the PWM signal generation circuits such that a gate control signalwith a higher operating frequency will be output to the gate drivecircuit corresponding to the power circuit that exhibits the lowesttemperature among the power circuit 227, 228 and 229.

The following details the allocation of carrier signals performed by thecarrier signal selection circuit 417. FIG. 24 is a flowchart showingprocedures for allocating carrier signals, performed by the carriersignal selection circuit 417.

On receipt of the temperature detection signals T1, T2 and T3 at StepS1, the carrier signal selection circuit 417 compares the temperaturesof the power circuits through the judgments at Step S2 and Step S3.

When the temperature indicated by the temperature detection signal T1 islower than the temperatures indicated by the temperature detectionsignals T2 and T3 (Step S2: YES), the carrier signal selection circuit417 outputs a carrier signal fc_2 to the PWM signal generation circuit414, and outputs carrier signals fc_1 and fc_3 to the PWM signalgeneration circuits 415 and 416, respectively (Step S3). The carriersignal fc_2 has the frequency of 20 kHz, and the carrier signals fc_1and fc_3 have the frequency of 10 kHz.

In the judgment at Step S2, when the temperature indicated by thetemperature detection signal T1 is not lower than the temperaturedetection signals T2 and T3 (Step S2: NO), the carrier signal selectioncircuit 417 compares the temperature detection signals T2 and T3. Whenthe temperature indicated by the temperature detection signal T2 islower than the temperature indicated by the temperature detectionsignals T3 (Step S4: YES), the carrier signal selection circuit 417outputs a carrier signal fc_2 to the PWM signal generation circuit 415,and outputs carrier signals fc_1 and fc_3 to the PWM signal generationcircuits 414 and 416, respectively (Step S5). The carrier signal fc_2has the frequency of 20 kHz, and the carrier signals fc_1 and fc_3 havethe frequency of 10 kHz.

In the judgment at Step S2, when the temperature indicated by thetemperature detection signal T2 is not lower than the temperatureindicated by the temperature detection signal T3 (Step S4: NO), thecarrier signal selection circuit 417 outputs a carrier signal fc_2 tothe PWM signal generation circuit 416, and outputs carrier signals fc_1and fc_3 to the PWM signal generation circuits 415 and 416, respectively(Step S6). The carrier signal fc_2 has the frequency of 20 kHz, and thecarrier signals fc_1 and fc_3 have the frequency of 10 kHz.

Through the procedures explained above, the one inverter with the powercircuit having the lowest temperature among the three-phase inverters221, 222 and 223 is provided with a gate control signal that is based onthe carrier signal having a higher frequency than the others.

Here, in the three-phase inverter to operate with a carrier signal of 20kHz, its power circuit generates a larger among of heat than thethree-phase inverters to operate with a carrier signal of 10 kHz. Thus,the relationship among the temperatures of the three-phase inverterswould change at some stage. However, the temperatures of the powercircuits in the three-phase inverters are measured cyclically, and theprocedures from Step S1 to Step S6 are repeated based on the latesttemperature detection signals T1, T2 and T3. Thus, with the structurepertaining to this embodiment, the three-phase inverter with the powercircuit exhibiting the lowest temperature at each stage is provided witha gate control signal that is based on a carrier signal having a higherfrequency than the other three-phase inverters.

In the embodiment as described above, concentration of the heat load tosome of the power circuits 227, 228 and 229 is prevented, which servesto improve the reliability of the system.

Generally, the iron loss of the stator coil to which the power isprovided by a three-phase inverter driving with a higher carrierfrequency is greater than the other stator coils, and thereforegenerates a larger amount of heat. However, in this embodiment, thethree-phase inverters to which the gate control signal that is based onthe carrier signal with the higher frequency is provided alternatessequentially. Thus, in the synchronous motor, the amounts of heatgenerated by the stator coils are equalized. Such an effect also servesto improve the reliability of the system.

Note that the operations of the control circuit 406 explained above maybe described with an application program and may be realized with amicrocomputer system performing the program.

<Modification of Embodiment 2>

As described above, the heat loads on the three-phase inverters 221, 222and 223 can be equalized by measuring the temperatures of the powercircuits of the three-phase inverters and supplying a gate controlsignal that is based on the carrier signal having the highest frequencyto the inverter with the lowest temperature. However, in view ofpreventing problems due to the heat load, it is not always necessary toequalize the heat loads on the three-phase inverters. That is, it isonly necessary that excessive heat generation such as heat more than anacceptable temperature level is prevented in the three-phase inverters.

For example, it is possible to avoid the case in which heat loadconcentration to a single three-phase inverter, which causes the risebeyond the acceptable temperature level, by cyclically alternating thedestination inverter among the three-phase inverters to which a gatecontrol signal that is based on the carrier signal with the highfrequency is supplied. In such a modification, it is preferable that thesupply destination of the gate control signal alternates in a cycle withwhich the excessive temperature of the three-phase inverter having beendriving with the high-frequency carrier signal decreases while anotherthree-phase inverter is driving with a high-frequency carrier signal.

Such a cycle is determined based on the structure of the invertermodule. For example, if three three-phase inverters are providedparallely within a module, the radiation performance of the three-phaseinverter in the middle is poorer than the other inverters on both sides.In view of this, it is preferable that the period in which a gatecontrol signal that is based on the high-frequency carrier signal issupplied to the three-phase inverter in the middle is shorter than thethree-phase inverters on both sides.

Alternatively, if all the three-phase inverters are capable ofperforming sufficient heat radiation in a short period, the destinationof the gate control signal that is based on the high-frequency carriersignal may be alternated among all the three-phase inverters at constantintervals.

The following describes a modification in which a gate control signalthat is based on the high-frequency carrier signal is supplied to thethree-phase inverters 221, 222 and 223 at constant intervals. FIG. 25 isa flowchart showing procedures for allocating carrier signals, performedby the carrier signal selection circuit 417 pertaining to thismodification.

The carrier signal selection circuit 417 includes a timer for outputtingtimeout signals at predetermined intervals, and a working memory.Firstly, the carrier signal selection circuit 417 sets 0 to the variableX on the working memory at Step S11, and repeats the loop from Step S12to Step S18.

In this loop, the carrier signal selection circuit 417 waits for thetimer to output a timeout signal (Step S12), and obtains a reminder leftwhen the variable X is divided by 3.

When the remainder is 0 (Step S13: YES), the carrier signal selectioncircuit 417 outputs a carrier signal fc_2 to the PWM signal generationcircuit 414, and outputs carrier signals fc_1 and fc_3 to the PWM signalgeneration circuits 415 and 416, respectively (Step S14). The carriersignal fc_2 has the frequency of 20 kHz, and the carrier signals fc_1and fc_3 have the frequency of 10 kHz.

When the remainder is 1 (Step S15: YES), the carrier signal selectioncircuit 417 outputs a carrier signal fc_2 to the PWM signal generationcircuit 415, and outputs carrier signals fc_1 and fc_3 to the PWM signalgeneration circuits 414 and 416, respectively (Step S16). The carriersignal fc_2 has the frequency of 20 kHz, and the carrier signals fc_1and fc_3 have the frequency of 10 kHz.

When the remainder is 2 (Step S15: NO), the carrier signal selectioncircuit 417 outputs a carrier signal fc_2 to the PWM signal generationcircuit 416, and outputs carrier signals fc_1 and fc_3 to the PWM signalgeneration circuits 414 and 415, respectively (Step S17). The carriersignal fc_2 has the frequency of 20 kHz, and the carrier signals fc_1and fc_3 have the frequency of 10 kHz.

Furthermore, the carrier signal selection circuit 417 increments thevariable X at Step S18 while repeating the loop.

Through such processing procedures, a gate control signal that is basedon the high-frequency carrier signal is supplied to the three-phaseinverters 221, 222 and 223 in this order, at the predetermined intervalsat which the timer outputs timeout signals.

Note that the intervals at which the timer outputs timeout signals maybe determined within the range where the temperature of the heatgenerated from the three-phase inverters does not rise beyond theacceptable temperature when gate control signals based on thehigh-frequency carrier signal are supplied continually.

Embodiment 3

FIG. 26 shows an overall structure of a synchronous motor drive systempertaining to Embodiment 3 of the present invention. The synchronousmotor drive system shown in FIG. 26 has the same structure as thesynchronous motor drive system shown in FIG. 1 except that the invertergroup 200 is replaced with an inverter group 230. The following explainsthe differences from the synchronous motor drive system pertaining toEmbodiment 1.

The inverter group 230 pertaining to this embodiment is characterized bygate drive circuits 231, 232 and 233 of the three-phase inverters.

The gate drive circuits 231, 232 and 233 have gate resistances betweentheir corresponding power circuits 207, 208 and 209 and them. Theresistance value of the gate resistance for the gate drive circuit 232,to which a gate control signal based on a 20 kHz carrier signal is inputfrom the PWM control unit 401, is lower than the gate drive circuits 231and 233 to which a gate control signal based on a 10 kHz carrier signalis input.

Thus, in the power circuit 208 that receives a gate drive signal fromthe gate drive circuit 232, the switching speed is higher than theothers because the channel of the switching device accumulates theelectrical charge quicker than the others. As a result, the switchingdevice in the power circuit 208 suffers less switching loss perswitching operation than the switching devices in the other powercircuits.

In the embodiment described above, in the PWM control for the invertergroup 230 based on a plurality of carrier signals, the switching lossper switching operation in the power circuit 208 that drives based onthe high-frequency carrier signal is lower than the other powercircuits. This keeps the balance among the switching losses that occurin the power circuits 207, 208 and 209 per unit time. Therefore,concentration of the heat load to some of the power circuits isprevented, which serves to improve the reliability of the system.

<Modification of Embodiment 3>

Combining Embodiment 2 with Embodiment 3 is highly effective.

In this embodiment, similarly to the Embodiment 2, the temperaturesensors measure the temperatures of the power circuits of thethree-phase inverters, and the PWM control unit outputs a gate controlsignal with use of 20 kHz carrier frequency to the gate drive circuitcorresponding to the power circuit whose measured temperature is thelowest, and outputs a gate control signal with use of 10 kHz carrierfrequency to the other two gate drive circuits.

Here, as shown in FIG. 27, each gate control circuit has a gateresistance 242 and a switch 243, which are parallely arranged between agate drive signal output unit 241 and the gate terminal of the powercircuit 207. The control circuit having such a structure outputs gatedrive signals by turning on the switch 243 when a gate control signalwith use of a 20 kHz carrier frequency is input from the PWM controlunit, and turning off the switch 243 when a gate control signal with useof a 10 kHz carrier frequency is input from the PWM control unit. As aresult, in the three-phase inverter to which the gate control signalwith use of the 20 kHz carrier frequency is input, the switching deviceof its power circuit performs the switching at a higher speed than theother three-phase inverters.

In the modification described above, in the PWM control for theinverters based on a plurality of carrier signals, the carrierfrequencies of the gate control signals supplied to the three-phaseinverters are switched based on the changes in the temperatures of thepower circuits, and the switching speed of the power circuit that drivesbased on the higher-frequency carrier signal than the others at themoment is set to be higher than the switching speeds of the other powercircuits. Therefore, concentration of the heat load to some of the powercircuits is prevented, which serves to improve the reliability of thesystem.

Embodiment 4

FIG. 28 shows an overall structure of a synchronous motor drive systempertaining to Embodiment 3 of the present invention. The synchronousmotor drive system shown in FIG. 28 has the same structure as thesynchronous motor drive system shown in FIG. 1 except that the positionestimating unit 403 is replaced with a position estimating unit 409. Thefollowing explains the differences from the synchronous motor drivesystem pertaining to Embodiment 1.

The position estimating unit 409 receives, among the three-phasealternate current detection signals detected by the current detectionmodule 500, the three-phase alternate current detection signals relatingto the three-phase inverter 201 detected by the current detectors 51, 52and 53, and calculates the inductance of the coils from the currentchange ratio per switching by the three-phase inverter 201. The positionestimating unit 409 estimates a rotor magnetic pole position θ of thesynchronous motor 300 from the inductance.

Here, the three-phase inverters 201, 202 and 203 operate according togate control signals with use of 10 kHz, 20 kHz and 10 kHz carriersignals, respectively. Generally, in PWM control for motors, theoperating frequency of the carrier signal corresponds to the period forcontrol calculation. Thus, in sensorless control for estimating themagnetic pole position θ, a longer control calculation period can beused for the sensorless control when the output from the three-phaseinverters that operate with the 10 kHz operating frequency is used thanwhen the output from the three-phase inverter that operates with the 20kHz operating frequency is used.

In the synchronous motor drive system pertaining to this embodiment asdescribed above, in the PWM control for the inverter group 230 based ona plurality of carrier signals, the magnetic pole position θ isestimated by using the output from the three-phase inverter that operatewith the lower carrier frequency. Thus, it is unnecessary to use aprocessing device having a high processing capability for the sensorlesscontrol. It is possible to reduce the cost by using a relatively cheapprocessing device. Moreover, in the case of increasing the frequency ofthe carrier signals along with the increase in number of the magneticpoles and rotation speed of motors, it is easy to perform the sensorlesscontrol in such motors by operating some of the three-phase inverterswith a low carrier frequency and estimating the magnetic pole position θby using the output from the three-phase inverter operating with the lowcarrier frequency.

Other Modifications

The synchronous motor drive system pertaining to the present inventionis described above based on the embodiments. However, the presentinvention is not limited to the embodiments. For example, the followingmodifications are possible.

(1) Embodiment 1 shows the case in which, among the three stator coilsshown in FIG. 3, the three-phase PWM voltage applied to the stator coils81 a and 83 a on both side is 10 kHz, and the three-phase PWM voltageapplied to the stator coil 82 a in the middle is 20 kHz. However, thepresent invention is not limited to this. Some effect can be expectedwith the structure in which one of the stator coil pair arranged alongthe circumference of the stator is supplied with a three-phase PWMvoltage based on a carrier signal with a lower frequency than the otherone.

FIG. 29 shows the waveforms of the actual motor currents (only U-phases)in the case where the operating frequencies of the three-phase PWMvoltages output from the three-phase inverters 201, 202 and 203 are 20kHz, 10 kHz and 20 kHz, respectively. FIG. 30 shows the waveforms of theactual motor currents (only U-phases) in the case where the operatingfrequencies of the three-phase PWM voltages output from the three-phaseinverters 201, 202 and 203 are 10 kHz, 20 kHz and 10 kHz, respectively.FIG. 31 shows the waveforms of the actual motor currents (only U-phases)in the case where the operating frequencies of the three-phase PWMvoltages output from the three-phase inverters 201, 202 and 203 are 20kHz, 20 kHz and 10 kHz, respectively. In any cases, the currents I_u1,I_u2 and I_u3 are the actual motor currents input to the stator coils 81a, 82 a and 83 a, respectively. In the example shown in FIG. 29, thecarrier signals, on which the input three-phase PWM voltages are basedon, have different frequencies between the pair of adjacent stator coils81 a and 82 a and between another pair of stator coils 82 a and 83 a. Inthe example shown in FIG. 30, the carrier signals, on which the inputthree-phase PWM voltages are based on, have different frequenciesbetween the pair of adjacent stator coils 81 a and 82 a. In the exampleshown in FIG. 31, the carrier signals, on which the input three-phasePWM voltages are based on, have different frequencies between the pairof adjacent stator coils 82 a and 83 a. In each pair of adjacent statorcoils, the change of magnetic flux caused by the current applied to thestator coil with the operating frequency of 20 kHz suppresses the changeof magnetic flux caused by the current applied to the stator coil withthe operating frequency of 10 kHz. This serves to suppress the increaseof the waveform distortion of the actual motor currents.

Also, due to the existence of the three-phase inverter that operatesbased on the 10 kHz carrier signal, the switching loss is reduced fromthe case where all the three-phase inverters operate with the frequencyof 20 kHz.

(2) In each embodiment, it is assumed that the number of the statorteeth included in a stator teeth group is three (m=3). However, thepresent invention is not limited to this, and any number is applicableas long as it is an integer no less than two. The following considersthe case where the number of the stator teeth included in the statorteeth group is m.

Logically, in the case where the number of the stator teeth is m, theadjacent stator teeth in the stator teeth group can be arranged at theinterval corresponding to (π+2π/3m) radians in electrical angle at themaximum. Also, in the case where the stator coils wound around theadjacent stator teeth generate magnetic fields having reversedpolarities when applied with currents in the same phase, currents indifferent phases within the range of ±2π/3m radians may be supplied tothe stator coils.

Practically, however, it is preferable that the adjacent stator teeth inthe stator teeth group are arranged at interval corresponding to(π+π/3m) radians in electrical angle at the maximum. Also, in the casewhere the stator coils wound around the adjacent stator teeth generatemagnetic fields having reversed polarities when applied with currents inthe same phase, currents in different phases within the range of ±π/3mradians may be supplied to the stator coils.

For example, when m=5, the number of stator teeth is 30 (i.e. equallyarranged every 12° in mechanical angle) and the number of the magneticpoles of the rotor is 32 (i.e. π radians in electrical angle is equal to360/32=11.25° in mechanical angle). Thus the adjacent stator coils arearranged at the intervals corresponding to the (π+π/15) radians inelectrical angle at the maximum. Also, one of the adjacent stator coilsthat is ahead of the other by (π+π/15) radians in electrical angle atthe maximum in the rotation direction is supplied with a current whosephase is (π+π/15) radians delayed from the other at the maximum.

Here, with the structure in which five three-phase inverters separatelysupply power to five stator coils arranged along the circumference ofthe stator, the three-phase inverter that supplies power to at least twoon both sides among the five stator coils is operated based on a 10 kHzcarrier signal, and the three-phase inverter that supplies power to thestator coil in the middle is operated based on a 20 kHz carrier signal.

(3) It is preferable that among the plurality of three-phase inverters,three-phase inverters operating with a substantially same carrierfrequency use carrier signals that are out of phase.

For example, as shown in FIGS. 32A-32C, the carrier signals fc_1 andfc_3 with the frequency of 10 kHz may be out of phase by ⅛ cycle.

(4) In the embodiments above, the magnetic pole position θ is estimatedby sensorless calculation. However, the magnetic pole position θ may bedirectly detected by using some means for detection. As a means fordetecting the magnetic pole position θ, a position detector such as anoptical encoder, a Hall sensor and a resolver may be used.

(5) In the embodiments above, both carrier signal generation circuit 411and 413 generate a 10 kHz carrier signal. However, the present inventionmay be embodied with carrier signal generation circuits 411, 412 and 413that each generate a carrier signal with a different frequency.

For example, the circuits 411, 412 and 413 may generate carrier signalswith the frequencies of 10 kHz, 20 kHz and 15 kHz, respectively. Whensuch a structure is adopted in Embodiment 2, it is preferable that theinverter whose power circuit has a lower temperature than the others isoperated with the lowest frequency carrier signal, and the inverterwhose power circuit has a higher temperature than the others is operatedwith the highest frequency carrier signal.

(6) The embodiments above exemplify synchronous motor drive systems eachincluding the same number of carrier signal generation circuits as thethree-phase inverters and the coils. However, the present invention maybe embodied with carrier signal generation circuits whose number isdifferent from the number of the three-phase inverters and the number ofcoils.

For example, in the case of a synchronous motor drive system includingthree three-phase inverters and two carrier signal generation circuits,two of the three-phase inverters are operated based on the carriersignal output from one of the carrier signal generation circuits, andthe other one of the three-phase inverters is operated based on thecarrier signal output from the other one of the carrier signalgeneration circuits.

Alternatively, the present invention may be embodied as a synchronousmotor drive system in which the number of carrier signal generationcircuits is larger than the number of three-phase inverters. Here, inthe case of a synchronous motor drive system in which the number ofcarrier signal generation circuits is larger than the number ofthree-phase inverters, the carrier frequencies to be allocated to thethree-phase inverters may be changed according to the drive status ofthe synchronous motor, such as the torque and the rotation speed.

Specifically, in a synchronous motor drive system including four carriersignal generation circuits for outputting carrier signals of 8 kHz, 10kHz, 15 kHz and 20 kHz and three three-phase inverters, when thesynchronous motor is driven at a high speed, two of the three-phaseinverters are operated with the 10 kHz carrier signal and the other oneof the three-phase inverters is operated with the 20 kHz carrier signal.When the synchronous motor is driven at a low speed, two of thethree-phase inverters are operated with the 8 kHz carrier signal and theother one of the three-phase inverters is operated with the 15 kHzcarrier signal.

With such a structure, the switching loss is expected to be furtherreduced when the motor is driven at a low speed.

(7) In the embodiments above, it is assumed, for example, that thehigher carrier frequency is 20 kHz and the lower carrier frequency is 10kHz. However, it is not necessary that the operating frequencies of thecarrier signals have a proportional relationship such as between 10 kHzand 20 kHz. In the present invention, it is preferable that one of theoperating frequencies of the carrier signals is not greater than twicethe other operating frequency.

(8) The embodiments above exemplify the cases where all the three-phaseinverters are provided with DC power from a single DC power source.However, as shown in FIG. 33, the present invention may be embodied witha structure in which each three-phase inverter is provided with DC powerfrom a different DC power source.

Here, the DC power sources 101, 102 and 103 shown in FIG. 33 may berealized with different kinds of power storage.

For example, when a three-phase inverter operates with a high carrierfrequency, the high-frequency noise is stronger than in the case with alow carrier frequency, which leads to generation of an excessive surgevoltage. In view of this, it is preferable that a fuel battery, which islighter but more susceptible to excessive voltage than a lead-acidbattery and the like, is used for supplying power to the three-phaseinverter that operates with the low carrier frequency, and a lead-acidbattery, a lithium-ion battery or the like is used for supplying powerto the three-phase inverter that operates with the high carrierfrequency.

Furthermore, each of the DC power sources 101, 102 and 103 shown in FIG.33 may supply power with a different voltage. The switching lossincreases as the voltage increases, and the high-frequency noiseincreases as well as the voltage increases. In view of this, it ispreferable that the voltage of the DC power source used for supplyingpower to the three-phase inverter that operates with the high carrierfrequency is lower than the voltage of the DC power source used forsupplying power to the three-phase inverter that operates with the lowcarrier frequency.

Note that as is the case with Embodiment 3, when the switching speed ofthe power device included in the three-phase inverter that operates withthe high carrier frequency is high, the increase of the high-frequencynoise along with the increase of the switching speed is problematic.However, such a problem is expected to be cancelled out to some extentby the effect of the high-frequency noise reduction due to the use of alow-voltage DC power source for supplying power to the three-phaseinverter that operates with the high carrier frequency, as describedabove.

(9) In each of the embodiments, the power circuits of the plurality ofthree-phase inverters may be configured with the same kind of switchingdevice, or with different kinds of switching devices.

Generally, the switching loss in unipolar devices, such asmetal-insulator-semiconductor field effect transistors (MISFET) andmetal-oxide-semiconductor field effect transistors (MOSFET) is lowerthan in bipolar devices, such as insulated gate bipolar transistors(IGBT). In view of this, the increase of the switching loss can besuppressed by using a unipolar device in the three-phase inverter thatoperates with the high carrier frequency and would cause the problem ofthe increase of the switching loss.

It is also possible to suppress the increase of the switching loss byusing a switching device with a wide-bandgap semiconductor in thethree-phase inverter that operates with the high carrier frequency andwould cause the problem of the increase of the switching loss. Awide-bandgap semiconductor, such as silicon carbide and gallium nitride,has a wider bandgap than Si semiconductors. If this is the case, inorder to suppress the increase of the cost, a switching device using aSi semiconductor, which is cheap, may be adopted in the three-phaseinverter that operates with the low carrier frequency.

(10) In the embodiments, the stator coils are wound around the statorteeth. However, the present invention is not limited to this. Thepresent invention is applicable to so-called coreless motors withoutstator teeth.

(11) The embodiments above exemplify the structure in which at least oneof the plurality of three-phase inverters is operated with a lowercarrier frequency than the other three-phase inverters so as to reducethe switching operations in number and reduce the switching loss.

The switching operation can also be suppressed by using, for the DC/ACconversion, a two-phase modulation method involving a pause period for60 degrees, for example, instead of by suppressing the carrierfrequency. Thus, a similar effect can be achieved in each of theembodiments by replacing the three-phase inverter that operates with thelow carrier frequency with a three-phase inverter that performs DC/ACconversion by a two-phase modulation method, and replacing thethree-phase inverter that operates with the high carrier frequency witha three-phase inverter that performs DC/AC conversion by a three-phasemodulation method. The present invention may be embodied with such astructure.

Note that even when all the three-phase inverters use a three-phasemodulation method or a two-phase modulation method, the presentinvention can be made applicable by causing at least one of theplurality of three-phase inverters to operate with a lower carrierfrequency than the others.

(12) Although not particularly mentioned in the description of theembodiments, a skew structure may be adopted for the stator coils. Witha skew structure, the stator coils are skewed with respect to the axisdirection of the rotor by, at the maximum, the interval between adjacenttwo of the stator coils.

(13) The embodiments above exemplify an outer rotor type synchronousmotor in which the rotor is disposed outside the stator. However, thesame effect can be achieved by an inner rotor type synchronous motor inwhich the rotor is disposed inside the stator, a so-called axial gaptype synchronous motor in which the rotor and the stator are disposedwith a space in between in the axial direction, and a synchronous motorwith a combination of these structures.

(14) In the embodiments above, the magnetic poles of the stator arecomposed of permanent magnets. However, the present invention isapplicable to synchronous motors using reluctance torque generated froma difference in magnetoresistance, and synchronous motors which includea combination of permanent magnets and reluctance torque in the rotor.

(15) Any of Embodiments 1 to 4 and the modifications may be combined.

INDUSTRIAL APPLICABILITY

The present invention realizes a synchronous motor drive system withhigh efficiency and low noise. Moreover, the present invention serves toreduce the cost by realizing sensorless control at a low cost. Thus, thepresent invention is applicable to any synchronous motor drive systemsthat are strongly demanded to be compact, including hybrid electricalvehicles, electrical vehicles, electrical compressors, electrical powersteering apparatuses and elevators, to power generation systems that arestrongly demanded to be compact as well, including wind power generatorsystems, and so on.

REFERENCE SIGNS LIST

2 rotor

3 stator

4 rotor core

5 permanent magnet

6 magnetic pole

9 stator coil

10 point between rotor magnetic poles

11 point between rotor magnetic poles

21 a-21 c coil terminals

22 a-22 c coil terminals

23 a-23 c coil terminals

24 a-24 c coil terminals

31 a-31 c coil terminals

32 a-32 c coil terminals

33 a-33 c coil terminals

34 a-34 c coil terminals

35 a-35 c coil terminals

36 a-36 c coil terminals

7 stator teeth

8 stator teeth group

8 a-8 c stator teeth groups

51-59 current detectors

61-63 temperature sensors

100-103 DC power sources

200, 210, 220, 230 inverter groups

201-203 three-phase inverters

204-206 gate drive circuits

207-209 power circuits

241 gate drive signal output units

242 gate resistance

243 switch

240 insulating substrate

250 resin mold

300, 304 synchronous motors

301-303 three-phase coils

400 control circuit

401 PWM control unit

402 current detector

403 position estimating unit

411-413 carrier signal generation circuits

414-416 PWM signal generation circuits

417 carrier signal selection circuit

500 current detection module

1. A synchronous motor drive system comprising: three-phase inverterseach configured to convert DC power to three-phase AC power; a controlcircuit configured to control operations of the three-phase inverters;and a synchronous motor configured to include three-phase coils suppliedwith three-phase AC power from the three-phase inverters, wherein thethree-phase inverters include first and second three-phase inverters,and the control circuit controls the operations of the three-phaseinverters by causing the first and the second three-phase inverters touse different carrier frequencies from each other to generatethree-phase AC power, each of the first and the second three-phaseinverters supplies a different one of the three-phase coils withthree-phase AC power, the synchronous motor has a stator in which statorcoils are arranged along a rotation direction of the synchronous motor,each of the stator coils constitutes one phase of one of the three-phasecoils, and the stator coils include a first stator coil and a secondstator coil arranged adjacent to each other, the first stator coilconstituting one phase of one of the three-phase stator coils that issupplied with three-phase AC power from the first three-phase inverter,the second stator coil constituting said one phase of another one of thethree-phase stator coils that is supplied with three-phase AC power fromthe second three-phase inverter.
 2. The synchronous motor drive systemof claim 1, wherein the synchronous motor further includes a rotorincluding magnetic poles arranged along the rotation direction at equalintervals, each of the stator coils is wound by concentrated winding,and every consecutive m of the stator coils belong to one of stator coilgroups arranged equiangularly, where m denotes an integer that is noless than 2, and at least two consecutive stator coils in each of thestator coil groups are each included in a different one of thethree-phase coils and individually supplied with three-phase AC powerfrom the first three-phase inverter and the second three-phase inverter.3. The synchronous motor drive system of claim 2, wherein the number ofthe three-phase inverters is three, and m is 3, the three-phaseinverters further include a third three-phase inverter, two at both endsof every consecutive three of the stator coils are individually suppliedwith three-phase AC power from the third three-phase inverter and thefirst three-phase inverter, one in the middle of every consecutive threeof the stator coils is supplied with three-phase AC power from thesecond three-phase inverter, and a carrier frequency difference betweenthe first three-phase inverter and the second three-phase inverter, anda carrier frequency difference between the second three-phase inverterand the third three-phase inverter are greater than a carrier frequencydifference between the first three-phase inverter and the thirdthree-phase inverter.
 4. The synchronous motor drive system of claim 3,wherein the control circuit causes the first three-phase inverter andthe third three-phase inverter to use a same carrier frequency togenerate three-phase AC power, and the carrier frequency used by thefirst three-phase inverter and the third three-phase inverter is lowerthan a carrier frequency used by the second three-phase inverter.
 5. Thesynchronous motor drive system of claim 2, wherein m is an odd number,the three-phase inverters further include a third three-phase inverter,one in the middle of every consecutive m of the stator coils is suppliedwith three-phase AC power from the second three-phase inverter, two ofevery consecutive m of the stator coils, both adjacent to the one in themiddle, are individually supplied with three-phase AC power from thethird three-phase inverter and the first three-phase inverter, and acarrier frequency difference between the first three-phase inverter andthe second three-phase inverter, and a carrier frequency differencebetween the second three-phase inverter and the third three-phaseinverter are greater than a carrier frequency difference between thefirst three-phase inverter and the third three-phase inverter.
 6. Thesynchronous motor drive system of claim 5, wherein the control circuitcauses the first three-phase inverter and the third three-phase inverterto use a same carrier frequency to generate three-phase AC power, andthe carrier frequency used by the first three-phase inverter and thethird three-phase inverter is lower than a carrier frequency used by thesecond three-phase inverter.
 7. The synchronous motor drive system ofclaim 1, wherein the synchronous motor further includes a rotorincluding magnetic poles arranged along the rotation direction at equalintervals, the stator includes stator teeth arranged along the rotationdirection, every consecutive m of the stator teeth belong to one ofstator teeth groups arranged equiangularly, where m denotes an integerthat is no less than 2, each of the stator teeth groups include a firststator tooth, a second stator tooth and a third stator tooth, the firststator tooth is wound with part of the first stator coil, the thirdstator tooth is wound with part of the second stator coil, the secondstator tooth is wound with the rest of the first stator coil and therest of the second stator coil, and the first stator coil and the secondstator coil are supplied with three-phase AC power from the firstthree-phase inverter and the second three-phase inverter, respectively.8. The synchronous motor drive system of claim 1, wherein a ratio of acarrier frequency used by the second three-phase inverter and a carrierfrequency used by the first three-phase inverter is no greater than 2 9.The synchronous motor drive system of claim 1, wherein the controlcircuit interchanges a carrier frequency used by the first three-phaseinverter with a carrier frequency used by the second three-phaseinverter, according to a predetermined condition, to generatethree-phase AC power.
 10. The synchronous motor drive system of claim 9,wherein each of the three-phase inverters includes a gate drive circuit,a power circuit corresponding to the gate drive circuit, and atemperature sensor for measuring a temperature of the power circuit, thecontrol circuit causes the first three-phase inverter or the secondthree-phase inverter, whichever includes the power circuit whosetemperature measured by the temperature sensor is higher, to use a lowercarrier frequency than the other to generate three-phase AC power. 11.The synchronous motor drive system of claim 10, wherein the gate drivecircuit included in the first three-phase inverter or the secondthree-phase inverter, whichever uses a higher carrier frequency togenerate three-phase AC power, drives the power circuit with a higherswitching speed than the other.
 12. The synchronous motor drive systemof claim 1, wherein each of the three-phase inverters includes a gatedrive circuit and a power circuit corresponding to the gate drivecircuit, and the gate drive circuit included in the first three-phaseinverter or the second three-phase inverter, whichever uses a highercarrier frequency to generate three-phase AC power, drives the powercircuit with a higher switching speed than the other.
 13. Thesynchronous motor drive system of claim 1 further comprising: a currentdetector configured to detect a value of alternating current output fromthe first three-phase inverter or the second three-phase inverter,whichever uses a lower carrier frequency to generate three-phase ACpower, and the synchronous motor further includes a rotor includingmagnetic poles, and the control circuit estimates positions of themagnetic poles based on the value detected by the current detector andcontrols the operations of the three-phase inverters according to thepositions.
 14. The synchronous motor drive system of claim 1, whereinthe control circuit changes, for each of the first and the secondthree-phase inverters, a carrier frequency to be used for generation ofthree-phase AC power, according to a drive state of the synchronousmotor.
 15. The synchronous motor drive system of claim 1, wherein aplurality of DC power sources are provided for supplying the three-phaseinverters with DC power, and each of the three-phase inverters issupplied DC power from a different one of the DC power sources.
 16. Thesynchronous motor drive system of claim 15, wherein at least two of theDC power sources have different overvoltage tolerances, and the firstthree-phase inverter or the second three-phase inverter, whichever usesa lower carrier frequency to generate three-phase AC power, is suppliedwith DC power from one of the power sources that has a low overvoltagetolerance.
 17. The synchronous motor drive system of claim 15, whereinat least two of the DC power sources supply DC power with differentvoltages, and the first three-phase inverter or the second three-phaseinverter, whichever uses a higher carrier frequency to generatethree-phase AC power, is supplied with DC power from one of the powersources that supplies DC power with a low voltage.
 18. The synchronousmotor drive system of claim 1, wherein each of the first three-phaseinverter and the second three-phase inverter includes a switching deviceof a different type, and the switching device included in the firstthree-phase inverter or the second three-phase inverter, whichever usesa higher carrier frequency to generate three-phase AC power, is aunipolar device.
 19. The synchronous motor drive system of claim 1,wherein each of the first three-phase inverter and the secondthree-phase inverter includes a switching device of a different type,the switching device included in the first three-phase inverter or thesecond three-phase inverter, whichever uses a lower carrier frequency togenerate three-phase AC power, is a Si semiconductor, and the switchingdevice included in the first three-phase inverter or the secondthree-phase inverter, whichever uses a higher carrier frequency togenerate three-phase AC power, is a wide-bandgap semiconductor with awider bandgap than the Si semiconductor.
 20. The synchronous motor drivesystem of claim 1, wherein the three-phase inverters include switchingdevices, and the switching devices are included in a single module. 21.The synchronous motor drive system of claim 20, wherein the moduleincludes three three-phase inverters aligned, and one in the middle ofthe three three-phase inverters includes, as a switching device, awide-bandgap semiconductor with a wider bandgap than a Si semiconductor.